Because mercury having high toxicity is included in flue gas discharged from a coal combustion boiler, which is a combustion device, for example, in a thermal power plant, various systems for removing mercury in flue gas have been conventionally studied.
Generally, a wet type desulfurizer for removing sulfur contents in flue gas is provided in the coal combustion boiler. In a flue-gas processing plant where a desulfurizer is attached to the boiler as an air pollution control apparatus, it is well known that if chlorine (Cl) contents in flue gas increase, the percentage of divalent metallic mercury (Hg) soluble in water increases, and thus the desulfurizer can easily collect mercury.
Recently, therefore, various processing methods and processing apparatuses of metallic mercury have been devised by combining NOx removal unit that reduces NOx and a wet type desulfurizer that uses an alkaline absorbent as a sulfur oxide (SOx) absorbent.
As a method of processing metallic mercury in flue gas, a removal method using an adsorbent such as activated carbon or a selenium filter has been known. However, this method requires a special adsorption removal unit, and thus it is not suitable for processing of large-capacity flue gas such as flue gas from a power plant.
Therefore, as a method of processing metallic mercury in large-capacity flue gas, there has been proposed a method such that a chlorinating agent is gas-atomized on an upstream side of NOx removal unit at a high temperature in a flue gas duct, mercury is oxidized (chlorinated) on a denitration catalyst to prepare soluble mercury chloride, and the mercury chloride is absorbed in a wet desulfurizer installed on a downstream side (see, for example, Patent Literatures 1 and 2). Further, an apparatus that atomizes gas to a flue gas duct and a technique therefor have been put to practical use in atomization of NH3 by NOx removal unit and gas atomization of the chlorinating agent.
FIG. 8 is a schematic diagram of an air pollution control system of a coal combustion boiler. As shown in FIG. 8, a conventional air pollution control system 100 includes a denitration catalyst layer 13 that removes nitrogen oxides (NOx) in flue gas 12 from a coal combustion boiler 11 that supplies coal as a fuel, and atomizes hydrochloric acid (HCl) into the flue gas 12 to oxidize mercury (Hg), an air preheater 14 that recovers heat in the flue gas 12 after removal of nitrogen oxides (NOx), an electronic precipitator 15 that removes dust in the flue gas 12 after heat recovery, a desulfurizer 16 that removes sulfur oxides (SOx) and mercury (Hg) in the flue gas 12 after dust removal, and a stack 18 that discharges the flue gas 12 that has undergone desulfurization to the outside as purged gas 17.
Further, an injection spot of hydrochloric acid (HCl) is provided in a flue gas duct 19 on an upstream side of the denitration catalyst layer 13, and hydrochloric acid (liquid) stored in a hydrochloric acid (liquid HCl) supplying unit 20 is gasified in a hydrogen chloride (HCl) atomizing unit 21 and atomized to the flue gas 12 as hydrogen chloride via a hydrogen chloride (HCl) atomizing nozzle 22.
Further, an injection spot of ammonia (NH3) is provided in the flue gas duct 19 on an upstream side of the denitration catalyst layer 13, and ammonia (NH3) supplied from an ammonia (NH3) supplying unit 23 is atomized to the flue gas 12 by an ammonia (NH3) atomizing nozzle 24, to reduce NOx.
In FIG. 8, reference numerals 25 and 26 denote an oxidation-reduction potential controller (ORP controller) and air, respectively.
The flue gas 12 from the boiler 11 is supplied to the denitration catalyst layer 13 and supplied to the electronic precipitator 15 after having heated air 27 by heat exchange in the air preheater 14, and further supplied to the desulfurizer 16, and then discharged to the air as the purged gas 17.
To suppress the influence of the chlorinating agent on an apparatus such as corrosive breakage and improve the reliability of the apparatus, the mercury concentration in flue gas, which has undergone wet desulfurization, is measured by a mercury monitor, and a feed rate of the chlorinating agent is adjusted based on the mercury concentration after desulfurization (see, for example, Patent Literature 2).
In this manner, conventionally, NOx in the flue gas 12 is removed and Hg in the flue gas 12 is oxidized by supplying hydrogen chloride and ammonia into the flue gas 12.
That is, NH3 is used for reduction and denitration of NOx, and NH3 supplied from the NH3 supplying unit 23 is atomized into the flue gas 12 via the NH3 atomizing nozzle 24. In the denitration catalyst layer 13, NOx is substituted by nitrogen (N2) by a reduction reaction as shown in the following equations, and then denitrated.4NO+4NH3+O2→4N2+6H2O  (1)NO+NO2+2NH3→2N+3H2O  (2)
Hydrogen chloride is used for mercury oxidation, and the hydrogen chloride used as the chlorinating agent is supplied from the liquid HCl supplying unit 20 to the HCl atomizing unit 21, where hydrochloric acid is gasified, and atomized into the flue gas 12 as HCl by the HCl atomizing nozzle 22. Accordingly, in the denitration catalyst layer 13, Hg having low solubility is oxidized (chlorinated) on the denitration catalyst as shown in the following equation, and converted to highly soluble mercury chloride (HgCl2), thereby removing Hg contained in the flue gas 12 by the desulfurizer 16 provided on a downstream side.Hg+2CHl+½O2→HgCl2+H2O  (3)
Further, when coal or heavy oil is used as a fuel, because Cl is contained in the fuel, combustion gas contains Cl components. However, the content of the Cl components in the fuel varies depending on the type of fuel, and thus it is difficult to control the Cl concentration in flue gas. Therefore, it is desired that HCl and the like in an amount more than required is added to the flue gas on an upstream side of an air pollution control apparatus 10 to remove mercury reliably.
Further, as the denitration catalyst layer 13, as shown in FIG. 7, a layer in which a denitration catalyst is supported on a honeycomb layer having square passages 28 arranged in a reticular pattern is used, and a cross-sectional shape of the passage is a multangular shape such as triangle or square.